Zinc guanidinate complexes and their application in ring-opening polymerisation catalysis

Article abstract of DOI:10.1002/ejic.200400031

The insertion reaction between 1 equiv. of carbodiimide, R'N=C=NR′, and the zinc bis(amide) [Zn(N(SiMe₃)₂)₂] affords the homoleptic zinc(II) guanidinate compounds [Zn{(Me ₃Si)₂NC(NR′)₂}₂] (3, R′ = iPr; 4, R′ = Cy) in preference to the monosubstituted compounds. Compound 3 also results from the protonation reaction of 1 equiv. of the neutral guanidine (Me₃Si)₂NC(NiPr)(NHiPr) (2) with [Zn{N(SiMe₃)₂}2] or ZnMe₂. In contrast, the N-dimethylguanidine analogue Me₂NC(NjPr)(NHiPr) (1) reacts cleanly with 1 equiv. of ZnMe₂ to afford the dimeric species [Zn{Me ₂NC(NiPr)₂}Me]₂ (5). Attempted protonation of the Zn-C bond in 5 with 2,6-ditert-butylphenol resulted in preferential reaction at the Zn-N bond and formation of the mixed (aryloxo)methyl complex [Zn(OAr)Me·{Me₂NC(NiPr)(NHiPr)}] (6), isolated as the guanidine adduct. Heating of 6, in an attempt to promote methane elimination, afforded a small amount of the dimetallic complex [{Zn(OAr)}₂(μ- {Me₂NC(NiPr)₂})(μ-NMe₂)] (7) as a result of the extrusion of NMe₂^- from the guanidine-based ligand. The amine-elimination reaction between [Zn{N(SiMe₃)₂} ₂] and 1 equiv. of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) proceeds smoothly to afford the mixed (guanidinate)(amide) complex [Zn(hpp){N(SiMe₃)₂}]₂ (8), which forms a dimer in the solid state. Crystal structures of compounds 3-8 are reported and a preliminary study of the activity of 8 in the ring-opening polymerisation of lactide is described. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400031

FULL PAPER
Zinc Guanidinate Complexes and Their Application in Ring-Opening
Polymerisation Catalysis
Martyn P. Coles*[a] and Peter B. Hitchcock[a]
Keywords: Guanidinate / N ligands / O ligands / Ring-opening polymerisation / Zinc
The insertion reaction between 1 equiv. of carbodiimide,
anidine adduct. Heating of 6, in an attempt to promote meth-
RЈN=C=NRЈ, and the zinc bis(amide) [Zn{N(SiMe3)2}2] affords
ane elimination, afforded a small amount of the dimetallic
the
homoleptic
zinc(II
)
guanidinate
compounds
complex [{Zn(OAr)}2(µ-{Me2NC(NiPr)2})(µ-NMe2)] (7) as a re-
sult of the extrusion of NMe2 from the guanidine-based
−
[Zn{(Me3Si)2NC(NRЈ)2}2] (3, RЈ = iPr; 4, RЈ = Cy) in preference
to the monosubstituted compounds. Compound 3 also results
from the protonation reaction of 1 equiv. of the neutral guani-
dine (Me3Si)2NC(NiPr)(NHiPr) (2) with [Zn{N(SiMe3)2}2] or
ZnMe2. In contrast, the N-dimethylguanidine analogue
Me2NC(NiPr)(NHiPr) (1) reacts cleanly with 1 equiv. of
ZnMe2 to afford the dimeric species [Zn{Me2NC(NiPr)2}Me]2
(5). Attempted protonation of the Zn−C bond in 5 with 2,6-di-
tert-butylphenol resulted in preferential reaction at the Zn−N
bond and formation of the mixed (aryloxo)methyl complex
[Zn(OAr)Me·{Me2NC(NiPr)(NHiPr)}] (6), isolated as the gu-
ligand.
The
amine-elimination
reaction
between
[Zn{N(SiMe3)2}2] and 1 equiv. of 1,3,4,6,7,8-hexahydro-2H-
pyrimido[1,2-a]pyrimidine (hppH) proceeds smoothly to af-
ford the mixed (guanidinate)(amide) complex [Zn(hpp)-
{N(SiMe3)2}]2 (8), which forms a dimer in the solid state. Crys-
tal structures of compounds 3−8 are reported and a prelimin-
ary study of the activity of 8 in the ring-opening polymeris-
ation of lactide is described.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004
containing a reactive ZnϪX bond, has been pursued in this
area, exhibiting varying degrees of activity and selectivity
dependant on the nature of the X group (Figure 1).[4,5] In
general, it has been found that compounds containing a
ZnϪOR group, especially when R is a small alkyl group
such as isopropyl, are significantly more active than those
containing the amide group [N(SiMe3)2]Ϫ, most likely due
to a sluggish insertion of the first monomer into the steri-
cally crowded ZnϪN linkage. A recent report has also
noted that structurally related bis(phosphanyl)methyl com-
plexes of zinc are also active for the ROP of lactide,[6] al-
though in these cases no evidence of ‘‘living’’ behaviour
was observed.
A related class of chelating N,NЈ-donor ligands that has
received attention in this area is the amidinate anion,
[RC(NRЈ)2]Ϫ,[7,8] where the metallacycle formed upon com-
plexation to a metal atom differs principally from β-diketi-
minate compounds in the number of constituent atoms (i.e.
4 vs. 5). A range of different metals have been investigated
including Fe,[9] Sn,[10] and the lanthanides[11] where, as for
the β-diketiminate complexes, the coordination sphere is
usually completed by either alkoxide or amide substituents
at which initial reaction with the monomer is considered to
take place (Figure 2). A notable exception is the series of
homoleptic lanthanide complexes [Ln{RC(NCy)2}3] (R ϭ
Me, Ln ϭ Nd, Gd, Yb; R ϭ Ph, Ln ϭ Nd, Yb),[12] which
are reported to be highly active for the ROP of ε-caprolac-
tone, presumably initiating at one of the MϪN
Introduction
There has been a great deal of recent interest in the me-
tal-catalysed ring-opening polymerisation (ROP) of cyclic
esters, in particular lactide (LA),[1] as benefits over poly-
mers derived from petrochemical sources include the renew-
able source of feedstock and the biocompatibility of the re-
sultant polyester material.[2] Well-defined metal catalysts
have generally allowed a greater control over the selectivity
of the polymerisation process to be achieved, and structure-
activity correlation studies associated with the ancillary li-
gands have been the focus of many of the recent develop-
ments in this area. As a consequence, many novel metal
alkoxide and amide species have been synthesised and
shown to be active catalysts for this process, where mono-
mer insertion into the metalϪoxygen or metalϪnitrogen
bond initiates polymerisation, and propagation ideally oc-
curs in a controlled fashion to furnish stereoregular polylac-
tide (PLA).
Anionic, N,NЈ-chelating ligands have thus far proved to
be one of the most promising classes of supporting group
in metal-catalysed lactide polymerisation. In particular, the
β-diketiminate anion,[3] in combination with a zinc centre
[a]
Department of Chemistry,
University of Sussex, Falmer, Brighton BN1 9QJ, UK
Fax: (internat.) ϩ 44-1273-677196
E-mail: m.p.coles@sussex.ac.uk
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Zinc Guanidinate Complexes
FULL PAPER
Figure 1. Zinc complexes, supported by β-diketiminate and related
ligands, that are active for the ring-opening polymerisation of lac-
tide
metalϪamidinate bonds. Extension of this area to include
the closely related guanidinate anions, [R2NC(NRЈ)2]Ϫ,[13]
has focussed mainly on the lanthanides, with catalytic ac-
tivity observed with examples of both mono-,[14] and bis(li-
gand)[15] complexes (Figure 3).
Figure 3. Metal complexes, supported by guanidinate ligands, that
are active for ring-opening polymerisation
Considering the potential for combining an active ZnϪX
(where X ϭ alkoxide, amide) component with a supporting
amidinate or guanidinate ligand set, it came as a surprise alkylzinc compounds and amidines to afford Zn(amidin-
to us that no accounts have appeared on the investigation ate)2 complexes appeared as unpublished results that, to the
of zinc amidinate and guanidinate complexes as catalysts in best of our knowledge, were never expounded upon.[7]
this area. Indeed, very few amidine or amidinate complexes
Recently, the synthesis of tolylformamidinate complexes
of zinc have been reported, with a few notable exceptions of zinc was investigated, although the only products forth-
being the [ZnBr2L2] adduct, where L ϭ neutral N,NЈ-di- coming from the study were cluster species incorporating
p-tolylformamidine,[16] and the bis(benzamidinate) complex oxygen.[18] Wheatley and co-workers have investigated
[Zn{PhC(NSiMe3)2}2].[17] A report of the reaction between mixed Zn/Li species containing N,NЈ-diphenylbenzamidin-
Figure 2. Metal complexes, supported by amidinate ligands, that are active for ring-opening polymerisation
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M. P. Coles, P. B. Hitchcock
FULL PAPER
ate and reported the reactivity with dry oxygen gas to afford
similar oligomeric structures.[19] In contrast, a recent review
did not report any zinc complexes containing anionic guan-
idinate ligands in the literature.[13] During the course of our
work, we have developed several areas of research based
around amidinate[20] and guanidinate chemistry,[21,22] and
wish to report the results from a study into the synthesis
and catalytic application of guanidinate-containing zinc
complexes.
Results and Discussion
Figure 4. Molecular structure of [Zn{(Me3Si)2NC(NiPr)2}2] (3)
An atom-efficient route that has been employed in the with thermal ellipsoids drawn at the 30% probability level; hydro-
gen atoms omitted for clarity
synthesis of metal guanidinate complexes is the insertion of
a carbodiimide into an existing metalϪamide bond,[14,23]
first reported by Lappert and co-workers in 1970.[24] The
stoichiometric reaction between RЈNϭCϭNRЈ (RЈ ϭ iPr,
Compounds 3 and 4 are monomeric in the solid state,
Cy) and [Zn{N(SiMe3)2}2] was therefore investigated as a with the metal coordination sphere consisting of two η2-
potential route to mono(guanidinato)zincϪamide com- guanidinate ligands in a ‘‘flattened’’ tetrahedral geometry.
plexes (Scheme 1). However, after the appropriate workup, The angles at the metal atom are in the range
the only product isolated from the reaction for each of the 66.23(8)Ϫ139.32(13)° and 66.03(12)Ϫ135.93(12)° for 3 and
carbodiimides investigated was the homoleptic, bis(guanid- 4, respectively, where the lowest value is a consequence of
inate) complex [Zn{(Me3Si)2NC(NRЈ)2}2] (3, RЈ ϭ iPr; 4, the small bite angle of the ligand. The four-membered met-
RЈ ϭ Cy), resulting from insertion of a molecule of carbodi- allacycles are essentially planar (largest deviation: 3, 0.015
˚
˚
imide into both zincϪamide bonds. Compounds 3 and 4 A; 4, 0 A) with an angle of 84.06° and 86.95° between the
can be purified by crystallisation from hexane at Ϫ30 °C ‘‘ZnNCN’’ least-squares planes for 3 and 4, respectively.
affording colourless crystals in moderate yield. The molecu- The carbonϪnitrogen distances are consistent with delocal-
lar structures are illustrated in Figures 4 and 5, crystal data isation across the ‘‘CN2’’ fragment of the metallacycle, and
˚
˚
are summarised in the Exp. Sect. and selected bond lengths the CϪN(SiMe3)2 distances [3, 1.428(3) A; 4, 1.430(5) A]
and angles in Table 1.
and large angles between the ‘‘CN2’’ and ‘‘NSi2’’ moieties
Scheme 1. Summary of the synthesis of zinc guanidinate complexes 3Ϫ7
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Zinc Guanidinate Complexes
FULL PAPER
(Scheme 1). In contrast, the reaction of the dimethylamino
analogue 1 with ZnMe2 afforded colourless crystals of 5
suitable for an X-ray diffraction study. The molecular struc-
ture is illustrated in Figure 6, crystal data are summarised
in the Exp. Sect. and selected bond lengths and angles in
Table 2.
Figure 5. Molecular structure of [Zn{(Me3Si)2NC(NCy)2}2] (4)
with thermal ellipsoids drawn at the 30% probability level; hydro-
gen atoms omitted for clarity
˚
Table 1. Selected bond lengths [A] and angles [°] for 3 and 4
3[a]
Figure 6. Molecular structure of [Zn{Me2NC(NiPr)2}Me]2 (5) with
thermal ellipsoids drawn at the 30% probability level; hydrogen
atoms omitted for clarity
ZnϪN(1)
2.011(2)
1.325(3)
1.326(3)
1.428(3)
66.23(8)
ZnϪN(2)
N(3)ϪSi(1)
N(3)ϪSi(2)
2.021(2)
1.753(2)
1.752(2)
C(1)ϪN(1)
C(1)ϪN(2)
C(1)ϪN(3)
N(1)ϪZnϪN(2)
N(1)ϪZnϪN(2Ј) 132.05(9)
N(1)ϪC(1)ϪN(2) 112.3(2)
N(1)ϪZnϪN(1Ј)
N(2)ϪZnϪN(2Ј)
134.89(12)
139.32(13)
˚
Table 2. Selected bond lengths [A] and angles [°] for 5 and 6
5[a]
4[b]
ZnϪN(1)
ZnϪN(3Ј)
C(2)ϪN(1)
C(2)ϪN(3)
2.101(3)
2.063(3)
1.393(6)
1.314(5)
1.357(5)
63.50(16)
130.9(2)
109.42(16)
ZnϪN(1Ј)
ZnϪC(1)
N(2)ϪC(6)
N(2)ϪC(7)
2.201(3)
1.987(5)
1.462(6)
1.461(6)
ZnϪN(1)
N(2)ϪSi
N(1)ϪZnϪN(1Ј)
N(1)ϪZnϪN(1Ј") 133.45(13)
2.006(2)
1.7590(15) C(1)ϪN(2)
66.03(12)
C(1)ϪN(1)
1.322(3)
1.430(5)
N(1Ј)ϪZnϪN(1Ј") 135.93(12)
N(1)ϪC(1)ϪN(1Ј) 111.5(3)
C(2)ϪN(2)
N(1Ј)ϪZnϪN(3Ј)
N(1Ј)ϪZnϪC(1)
N(3Ј)ϪZnϪN(1)
N(1Ј)ϪZnϪN(1)
N(3Ј)ϪZnϪC(1)
N(1)ϪZnϪC(1)
91.80(12)
126.05(18)
119.74(19)
[a]
[b]
Symmetry elements for 3: Ј Ϫx ϩ 1, y, Ϫz ϩ 1/2.
Symmetry
elements for 4: Ј Ϫx ϩ 5/4, y, Ϫz ϩ 1/4; ЈЈ Ϫx ϩ 5/4, Ϫy ϩ 1/4, z;
ЈЈЈ x, Ϫy ϩ 1/4, Ϫz ϩ 1/4.
N(1)ϪC(2)ϪN(3) 112.2(4)
6
[3, 87.61°; 4, 81.46°] are consistent with a single bond with
possible NϪSi π-interactions.[25]
ZnϪO
1.9511(15) ZnϪC(24)
1.9952(18) C(1)ϪN(1)
1.955(2)
1.324(3)
1.373(3)
2.83
107.78(7)
110.96(12)
114.34(13)
An alternative approach to the targeted mono(guanidin-
ate) complexes was subsequently investigated, namely the
reaction between dimethylzinc and 1 equiv. of the (neutral)
guanidine, predicted to proceed by protonolysis of
the zincϪcarbon linkage. The ligand precursors,
[R2NC{NRЈ}{NHRЈ}], were prepared from the reaction
between the lithium diamide LiNR2 (R ϭ SiMe3, Me) and
iPrNϭCϭNiPr, followed by quenching of the intermediate
lithium salt with a stoichiometric amount of water, to afford
[Me2NC(NiPr)(NHiPr)] (1) and [(Me3Si)2NC(NiPr)-
ZnϪN(1)
C(1)ϪN(2)
OϪC(10)
OϪZnϪC(24)
N(1)ϪZnϪC(24) 131.30(9)
C(1)ϪN(1)ϪZn 125.04(15)
1.353(3)
1.358(3)
120.79(9)
C(1)ϪN(3)
H(2A)···O
OϪZnϪN(1)
ZnϪOϪC(10)
C(2)ϪN(1)ϪZn
C(1)ϪN(1)ϪC(2) 119.45(18)
C(1)ϪN(3)ϪC(9) 121.6(2)
C(1)ϪN(3)ϪC(8) 123.0(2)
C(8)ϪN(3)ϪC(9) 115.1(2)
[a]
Symmetry elements for 5: Ј Ϫx, Ϫy ϩ 1, z.
Compound 5 crystallised as the dimeric compound
(NHiPr)] (2).[26] The compounds were isolated as colourless [Zn{Me2NC(NiPr)2}Me]2, with
a
µ,η2:η1-coordination
and yellow liquids, respectively, and were used without mode of the guanidinate ligand, resulting in an essentially
further purification. The reaction between
2
and planar Zn2N2 ring (largest deviation from least-squares
˚
[Zn{N(SiMe3)2}2] or ZnMe2 in a 1:1 ratio did not proceed plane: 0.041 A). Whilst this type of coordination has not
to the mono(ligand) complex, but afforded only the bis(gu- previously been observed for monoanionic guanidinate li-
anidinate) species
3
after the appropriate workup gands,[27] related tricyclic core structures have been reported
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M. P. Coles, P. B. Hitchcock
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for amidinate complexes of lithium,[28Ϫ30] magnesium[31]
and iron,[32] and we have also observed this structural motif
in a recently reported lithium phospha()guanidinate com-
plex.[33] In contrast to the majority of these structures
which contain crystallographic inversion symmetry, a non-
centrosymmetric structure is observed for compound 5,
with a syn arrangement of the ligands about the central
Zn2N2 ring.[29,31] The zinc atoms are present in a distorted
tetrahedral
geometry
[angles
in
the
range
63.50(16)Ϫ130.9(2)°], with the ‘‘ZnNCN’’ metallacycle at
an angle of 66.19° to the central Zn2N2 plane. Examination
of the ZnϪN bond lengths in 5 suggests a strong associ-
ation of the monomeric units in the solid state. Indeed, if
we consider the dimer as consisting of two [Zn(guanidinate)-
Me] fragments, based solely upon metalϪnitrogen dis-
tances there is an apparent stronger interaction between the
Figure 7. Molecular structure of [Zn(OAr)Me·{Me2NC-
(NiPr)(NHiPr)}] (6) with thermal ellipsoids drawn at the 30% prob-
ligand of one unit and the metal of another [ZnϪN(1) ϭ ability level; hydrogen atoms, except H(2a), omitted for clarity
˚
2.101(3) A], than within the chelating guanidinate itself
˚
[ZnϪN(1Ј) ϭ 2.201(3) A]. In all cases the ZnϪN distances
are significantly longer than in the homoleptic compounds shortening may also be enhanced by the intramolecular H-
˚
˚
˚
3 (av. 2.016 A) and 4 (2.006 A), despite a reduced angle bond noted above. The ZnϪO bond [1.9511(15) A] is rela-
between the NR2 plane and the metallacycle in 5 (31.60°) tively long in comparison to related three- and four-coordi-
that allows contribution from a zwitterionic resonance to nate zinc complexes containing this and the similarly substi-
the overall bonding.[25] In agreement with this postulate, the tuted
2,4,6-tBu3C6H2
aryloxide
groups
[range:
˚
˚
C(2)ϪN(2) bond length of 5 [1.357(5) A] is intermediate 1.818(1)Ϫ1.923(5) A], and the angle at the oxygen atom
between the value expected for CϪN single and CϭN [110.96(12)°] is by far the smallest observed [range:
double bonds, and is significantly shorter than the corre- 124.04(14)Ϫ155.7(10)°].[35] These data suggest that there is
˚
˚
sponding bonds in 3 [1.428(3) A] and 4 [1.430(5) A].
a reduced π-donation from the oxygen to the zinc atom in
Transformation of the zincϪmethyl group to a ZnϪO 6 compared with the other examples cited, caused by the
linkage, desirable for applications in ROP, was attempted localisation of electron density in the hydrogen bond to the
by reaction with 2,6-di-tert-butylphenol (Scheme 1). It was amino NH group.
anticipated that the steric bulk of the alkyl substituents po-
The EIϩ mass spectrum of 6 indicates that, as predicted
sitioned about the resultant metalϪoxygen bond would dis- under these experimental conditions, dissociation of the
favour a bridging coordination mode for the aryloxide and guanidine occurs resulting in a signal at m/z (%)ϭ 284 (72)
promote a low aggregation within the final product. The equivalent to the ‘‘Zn(OAr)Me’’ fragment. Slightly more
reaction was performed at room temperature and after the unexpected was the observation of a peak at m/z (%) ϭ 439
appropriate workup colourless crystals of 6 were obtained. (38) in the same spectrum, corresponding to the monomeric
The molecular structure is illustrated in Figure 7, crystal form of the target complex, ‘‘Zn{Me2NC(NiPr)2}(OAr)’’.
data are summarised in the Exp. Sect. and selected bond This fragment must arise as a result of protonation of the
lengths and angles in Table 2.
methyl group by the NH functionality, with loss of meth-
The molecular structure of 6 shows retention of the ane, and prompted us to investigate the thermal stability
zincϪmethyl group and incorporation of the aryloxide li- of 6 in the expectation that, under the correct conditions,
gand in an overall monomeric, distorted trigonal planar reactivity could be promoted at the ZnϪC bond. Thus, a
zinc complex (Σangles at Zn: 359.57°). The coordination mixture of 5 and 1 equiv. of HOAr was heated in toluene,
sphere of the zinc atom is completed by the neutral form of and the resultant solid crystallised from hexane at Ϫ30 °C,
the guanidine ligand, Me2NC(NiPr)(NHiPr) (1), bonding affording a small number of colourless crystals of 7 that
through the Nimino atom,[34] such that the ‘‘CN2’’ compo- were analysed by NMR spectroscopy and X-ray diffraction.
nent is essentially coplanar with the ZnMeON metallacycle The X-ray diffraction study was performed on representa-
˚
[max. deviation at C(1) 0.15 A]. This arrangement is likely tive crystals of 7 to determine the nature of the product;
governed by the formation of an intramolecular hydrogen the molecular structure is illustrated in Figures 8 and 9,
bond between the amino-NH and the aryloxide-O atoms crystal data are summarised in the Exp. Sect. and selected
˚
[H(2a)···O 2.83 A] which results in a widening of the angle bond lengths and angles in Table 3.
between the N(1) and C(24) atoms at the zinc atom from
Compound 7 is a dimetallic zinc complex where the metal
the ideal trigonal planar angle to 131.30(9)°.
atoms are joined by bridging guanidinate and amide groups
Despite coordinating as a neutral ligand in 6, the ZnϪN to generate a six-membered metallacycle. Each zinc atom is
˚
bond [1.9952(18) A] is shorter than those observed in 5 (av. also bound by an aryloxide ligand, giving an overall dis-
˚
2.12 A) where it binds as an anionic ligand, reflecting the torted trigonal-planar geometry with the sum of the angles
reduction in coordination number at the metal centre. This at Zn(1) and Zn(2) equal to 359.54° and 359.37°, respec-
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Zinc Guanidinate Complexes
FULL PAPER
˚
Table 3. Selected bond lengths [A] and angles [°] for 7 and 8
7
Zn(1)ϪN(2)
Zn(1)ϪO(1)
Zn(1)ϪN(4)
C(1)ϪN(1)
C(1)ϪN(3)
N(4)ϪC(39)
O(1)ϪZn(1)ϪN(2) 135.79(12)
O(1)ϪZn(1)ϪN(4) 111.24(12)
N(2)ϪZn(1)ϪN(4) 112.51(13)
N(1)ϪC(1)ϪN(2) 119.3(3)
1.950(3)
1.846(2)
1.990(3)
1.352(5)
1.361(5)
1.461(6)
Zn(2)ϪN(1)
Zn(2)ϪO(2)
Zn(2)ϪN(4)
C(1)ϪN(2)
N(4)ϪC(38)
1.946(3)
1.840(2)
1.982(3)
1.336(5)
1.470(6)
O(2)ϪZn(2)ϪN(1) 134.73(12)
O(2)ϪZn(2)ϪN(4) 111.50(13)
N(1)ϪZn(2)ϪN(4) 113.14(13)
8[a]
Figure
8.
Molecular
structure
of
[{Zn(OAr)}2(µ-
ZnϪN(1)
ZnϪN(1Ј)
C(1)ϪN(1)
C(1)ϪN(2)
C(1)ϪN(3)
N(1)ϪZnϪN(2)
N(1)ϪZnϪN(4)
2.1879(18) ZnϪN(2)
2.0831(19) ZnϪN(4)
2.0486(19)
{Me2NC(NiPr)2})(µ-NMe2)] (7) with thermal ellipsoids drawn at
the 20% probability level; hydrogen atoms omitted for clarity
1.9177(18)
1.707(2)
1.702(2)
1.380(3)
1.318(3)
1.349(3)
64.20(7)
131.93(8)
N(4)ϪSi(1)
N(4)ϪSi(2)
N(1)ϪZnϪN(1Ј)
N(2)ϪZnϪN(4)
92.88(7)
125.11(8)
N(2)ϪZnϪN(1Ј) 104.61(7)
N(1Ј)ϪZnϪN(4) 122.41(8)
N(1)ϪC(1)ϪN(2) 113.37(19)
ZnϪN(1)ϪZnЈ
87.12(7)
[a]
Symmetry elements for 8: Ј Ϫx ϩ 1, Ϫy, Ϫz ϩ 1.
associated with this substituent (vide supra), although the
angle at the oxygen atom varies significantly between
the
two
ligands
[Zn(1)ϪO(1)ϪC(10)
130.0(2)°,
Zn(2)ϪO(2)ϪC(24) 140.9(2)°] indicating the ‘‘soft’’ charac-
ter of this angle.
Recently, we[21,22] and others[25,40] have utilised the com-
mercially available bicyclic guanidine 1,3,4,6,7,8-hexahydro-
2H-pyrimido[1,2-a]pyrimidine (hppH) as a ligand precursor
in the coordination chemistry of transition-metal and main-
group complexes. The major difference of this system in
comparison with acyclic guanidinates is that the constraint
of the amide substituent and the amidine ‘‘CN2’’ compo-
nents of the molecule in the cyclic framework is predicted
Figure 9. Core of [{Zn(OAr)}2(µ-{Me2NC(NiPr)2})(µ-NMe2)] (7)
along the approximate N(4)ϪC(1)ϪN(3) vector, illustrating the
‘‘half-chair’’ geometry of the metallacycle, and the distribution of
the ligand substituents
tively. Overall, the metallacycle adopts a half-chair confor- to enhance electron donation to a metal centre by increas-
mation (Figure 9) with a twist angle of 34.56° between ing the contribution to the overall bonding from a zwit-
the planes defined by Zn(1)ϪN(4)ϪZn(2) and terionic resonance form. A potential complication in the
N(1)ϪC(1)ϪN(2). Both the guanidinate and amide bridge application of the anionic ligand [hpp]Ϫ in molecular ca-
symmetrically between the two metal atoms.
talysis that arises from the bicyclic nature of the ligand is
Whilst a number of examples of bridging acyclic guanidi- the propensity to adopt a bridging coordination mode to
nates have been reported between Pd, Fe and the s-block different metal centres, extensively exploited by Cotton and
metals,[36] by far the most common coordination mode in- others in the support of many dimetallic systems.[30,41] In-
volves chelating to the metal atom,[13] rationalised in terms deed, we have observed that the stoichiometric reaction be-
of the ligand donor orbital pointing towards the ‘‘mouth’’ tween neutral hppH and ZnMe2 affords the fully character-
of the ligand. The CϪN distances within the guanidinate ised, trimetallic complex [Zn3(hpp)4Me2] containing both
ligand (av. 1.35 A) signify delocalisation of the R2N lone two-coordinate [µ-hpp]Ϫ and three-coordinate [κ1,2N-κ3NЈ-
˚
pair into the core of the ligand, typically observed for this hpp]Ϫ bonding modes; this will be described fully in a forth-
substitution pattern of nitrogen substituents. The average coming publication.[42]
˚
ZnϪNamide distance in 7 (1.963 A) is shorter than that
In contrast to the reactions between acyclic guanidines
found to the bridging amide group in the homoleptic series and [Zn{N(SiMe3)2}2], the reaction with hppH proceeds
[38]
[Zn(NR2)(µ-NR2)]2 (R ϭ Ph,[37] 2.03 A; R ϭ CH2Ph,
smoothly at room temperature to afford the mono(guanid-
˚
[39]
˚
˚
2.04 A; R ϭ CH2iPr, 2.03 A], a possible consequence of inate) species [Zn(hpp){N(SiMe3)2}]n (8) in excellent yield.
the greater electron-withdrawing ability of the ArOϪ ligand. Low-temperature (Ϫ30 °C) crystallisation from a concen-
˚
The ZnϪO bond lengths (av. 1.843 A) are within the range trated hexane solution affords colourless crystals suitable
Eur. J. Inorg. Chem. 2004, 2662Ϫ2672
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2667

M. P. Coles, P. B. Hitchcock
FULL PAPER
for an X-ray diffraction study. The molecular structure is
Preliminary NMR investigations of the catalytic activity
illustrated in Figure 10, crystal data are summarised in the of 8 towards the ring-opening of ,-lactide (LA) was per-
Exp. Sect. and selected bond lengths and angles in Table 3. formed in CD2Cl2 and [D8]toluene. In contrast to many
zinc complexes incorporating the relatively bulky N(SiMe3)2
group, where initiation is reported as being slow,[4] the im-
mediate reaction of 8 to generate a new species (Ͼ 98% in
ca. 2 min) was observed, as judged by a shift of the
N(SiMe3)2 resonance from δ ϭ Ϫ0.02 ppm to δ ϭ 0.05 ppm
(CD2Cl2). The progress of the reaction was monitored by
1H NMR spectroscopy, clearly indicating that 45Ϫ85 equiv.
of LA were polymerised to polylactide (PLA) with more
than 95% conversion in less than 2 h. Assuming the rate of
initiation is much greater than the rate of propagation, lead-
ing to all zinc centres being activated prior to further inser-
tion of monomer, integration of the N(SiMe3)2 end-group
signal versus either the methine or methyl resonances of the
growing polymer chain will allow an approximate value of
1
the molecular weight to be calculated. From the H NMR
spectra of the polymerisation reactions performed with 8,
this appeared to be a reasonable approximation (vide su-
pra), and a graph of molecular weight vs. conversion was
therefore plotted (Figure 11). The straight-line relationships
generated for different monomer concentrations and sol-
vents present good evidence that the polymerisation pro-
ceeds in a ‘‘well-behaved’’ fashion. In addition, after 18 h
Figure 10. Molecular structure of [Zn(hpp){N(SiMe3)2}]2 (8) with
at room temperature, a sample of 8 and LA (45 equiv.) that
thermal ellipsoids drawn at the 30% probability level; hydrogen
had progressed to more than 98% conversion was able to
initiate the polymerisation of a further 50 equiv. of LA in
CD2Cl2. The fresh monomer was converted into PLA over
a period of 2 h, demonstrating the stability of the propagat-
atoms omitted for clarity
Compound 8 crystallised as the dimeric compound ing species over time and providing further indication of
[Zn(hpp){N(SiMe3)2}]2, with
a previously unreported the controlled nature of the polymerisation. There was no
1
µ,η2:η1-coordination mode for the [hpp]Ϫ anion. Unlike the evidence for a tacticity bias in the H NMR spectrum of
molecular structure of 5, complex 8 exists as a centrosym- the PLA produced by 8, as indicated by selective decoupling
metric dimer with an anti arrangement of ligands about the of the methine protons.
central Zn2N2 metallacycle, a likely consequence of the dif-
ferent steric requirements of the ligands at the zinc centre.
The Zn2N2 core of the molecule is perfectly planar and
forms an angle of 72.9° to the essentially planar ZnN2C
metallacycle (largest deviation from least-squares plane
˚
0.075 A). The resultant geometry at the zinc centre is there-
fore best described as distorted tetrahedral with bond
angles in the range 64.20(7)° (corresponding to the bite an-
gle of the ligand) to 131.93(8)°.
As in 5, a strong interaction between the two ‘‘mono-
meric’’ [Zn(hpp){N(SiMe3)2}] units is indicated by the
˚
shorter ZnϪN(1Ј) distance [2.0831(19) A] compared with
˚
ZnϪN(1) [2.1879(18) A]. The remaining ZnϪN bond of the
chelating [hpp]Ϫ anion is shorter [2.0486(19) A], a probable
˚
combination of increased electron donation from [hpp]Ϫ
[angle between N(1)ϪC(1)ϪN(2) and C(4)ϪN(3)ϪC(5) termined by H NMR spectroscopy) for the polymerisation of LA
Figure 11. Plot of molecular weight vs. monomer conversion (de-
1
with 8
planes 4.57°] and a more electron-deficient zinc atom aris-
ing from the presence of the amide vs. the methyl substitu-
˚
ent. The ZnϪNamide distance [1.9177(18) A] is slightly
longer than the value in related, three-coordinate β-diketim-
Conclusion
inate[43] and (imine)(oxazoline)[44] zinc amides [1.896(2) and
˚
1.883(3) A, respectively], which may reflect a weaker bond,
more reactive to ROP.
We have demonstrated that the synthesis of mono(guani-
dinate) complexes of zinc is beleaguered with difficulties,
2668
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Eur. J. Inorg. Chem. 2004, 2662Ϫ2672

Zinc Guanidinate Complexes
FULL PAPER
and the nature of the product can be highly dependant on idinate and retention of the aryloxide substituent. However,
the synthetic protocol employed and substitution pattern of the guanidinate ligand was found to be unstable under the
the nitrogen groups contained within the ligand precursor. conditions employed, as indicated by the isolation of a
Initial attempts at formation of the mono(ligand) amide small amount of the dimetallic species [{Zn(OAr)}2(µ-
only afforded the homoleptic ZnII guanidinate products 3 {Me2NC(NiPr)2})(µ-NMe2)] (7), formed by amide ex-
and 4 under a variety of conditions. Assuming a stepwise trusion from the guanidine or guanidinate ligand. As this
reaction pathway is in operation, proceeding by initial for- product was only isolated in low yield, and as such may
mation of one guanidinate anion at the metal atom, this represent only a minor component of the product mixture,
mono(ligand) product evidently activates the metalϪamide detailed mechanistic studies into its formation were not car-
bond to further insertion relative to the ZnϪN bond of the ried out. However further work in this area is merited due
bis(amide) starting material.
to the possible implications that such a process has in me-
An interesting contrast was noted in the protonation re- tal-catalysed guanylation of amines and transamination of
activity between ZnMe2 and the neutral diisopropylguani- guanidines,[45] and CϭN metathesis of carbodiimides.[46]
dines R2NC(NiPr)(NHiPr) (R ϭ SiMe3 and Me), where a
Building on the observation that the introduction of a
similar steric environment at the metal atom is anticipated more electron-donating guanidinate ligand caused multiple
as the metal-bound nitrogen atoms are substituted by iso- protonation at the metal centre to be effectively ‘‘shut
propyl groups in each instance. Changing the nitrogen sub- down’’ during the synthesis of 5, we turned our attention
stituents of the NR2 moiety from trimethylsilyl to methyl, to the potentially more strongly electron-donating anion
however, arrested multiple protonation of the dialkylzinc [hpp]Ϫ. It has been shown that constraining the NR2 and
and provided a clean and efficient route to the first example CN2 components in the bicyclic framework of this com-
of a mono(guanidinate) compound 5 of zinc. It has been pound promotes an increased contribution from a zwit-
previously demonstrated in guanidinate chemistry of alu- terionic resonance form, manifested in shorter TiϪN dis-
minium that, despite being remote from the metal centre, tances in a series of titanium compounds incorporating the
the nature of the R substituents can strongly influence the [hpp]Ϫ anion in comparison to acyclic amidinates and gu-
distribution of electron density within the [R2NC{NRЈ}2]Ϫ anidinates.[21] Thus, synthesis of [Zn(hpp){N(SiMe3)2}]2 (8),
anion and the potential therefore exists to ‘‘tune’’ the reac- containing the desired combination of a supporting ancil-
tivity of the metal centre.[25] In general, the larger, more lary guanidinate and reactive zincϪamide bond, was
electron-withdrawing silyl substituents will reduce the ex- achieved. Compound 8 was found to be active in the small-
tent of electron donation through the NaminoϪC bond by scale polymerisation of lactide and analysis of the NMR
disfavouring delocalisation of the Namino lone-pair into the spectroscopic data indicated a well-behaved system.
framework of the ligand. The smaller, electron-donating
Overall we have therefore exploited the observation that,
methyl groups, however, allow the NR2 and CN2 compo- in order to prevent the formation of homoleptic ZnII guani-
nents of the ligand to approach coplanarity, thus providing dinate compounds, more electron-donating guanidinate
the correct orbital alignment for interaction between the anions must be employed. Eventually, the correct combi-
nitrogen lone-pair and the empty p-orbital of the sp2-car- nation of ligands was found in 8 and we proved the concept
bon atom. The result is the potential for greater electron that this compound is an active catalyst for the well-be-
donation for the anion derived from 1 compared with 2, haved ROP of lactide. The application of 8 in the polymeris-
which is apparently sufficient to prevent activation of the ation of related monomers and an in-depth study of the
zincϪmethyl bond to further protonation in compound 5. structure of the polymeric material will be published in
It is curious to note that in 5, the usually highly reactive due course.
ZnϪCH3 bond is even resistant to protonation from a
bulky phenol derivative and that the acidic proton preferen-
tially reacts at one of the ZnϪN bonds of the guanidinate,
affording the mixed methyl(aryloxide) compound 6.
Experimental Section
The molecular structure of 6 confirms retention of the
neutral ligand at the zinc centre and reveals an intramolecu-
lar hydrogen bond to the aryloxide-O atom. Comparison of
General Remarks: All manipulations were carried out under dry
nitrogen using standard Schlenk and cannula techniques, or in a
conventional nitrogen-filled glovebox operating at Ͻ 1 ppm oxygen.
the ZnϪO bond with other examples of compounds con- Solvents were dried with appropriate drying agents, degassed, and
stored over a potassium mirror or activated molecular sieves prior
to use. RЈNϭCϭNRЈ (RЈ ϭ iPr, Cy; Aldrich), LiNMe2 (Fluka),
LiN(SiMe3)2 (Aldrich) and ZnMe2 (ca. 2  solution in toluene;
Fluka) were obtained commercially and used as received, unless
stated otherwise. The compound [Zn{N(SiMe3)2}2] was synthesised
from the reaction between ZnBr2 and 2 equiv. of LiN(SiMe3)2. The
crude product was extracted into hexane and used without further
purification. Elemental analyses were performed by S. Boyer at The
University of North London. NMR spectra were recorded with a
Bruker Avance DPX 300 MHz spectrometer. Coupling constants
taining the same ligand indicates that the bond in 6 is ex-
ceptionally long and angular. This is in agreement with the
oxygen lone-pairs participating in the hydrogen bond to the
NH group, with the result being consistent with a lower
ZnϪO bond order than noted in previous examples of zinc
complexes incorporating this ligand.
Attempts to promote methane elimination by heating a
mixture of 5 and ArOH met with partial success. Loss of
the zincϪmethyl group was achieved with concomitant con-
version of the neutral guanidine ligand to the anionic guan- are quoted in Hertz.
Eur. J. Inorg. Chem. 2004, 2662Ϫ2672
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2669

M. P. Coles, P. B. Hitchcock
FULL PAPER
Me2NC(NiPr)(NHiPr) (1): A slurry of lithium dimethylamide
(5.00 g, 98.00 mmol) in Et2O (100 mL) was cooled to 0 °C and a
solution of diisopropylcarbodiimide (12.40 g, 98.00 mmol) in Et2O
(50 mL) was added dropwise with a cannula. The mixture was al-
lowed to warm to room temperature to afford a cloudy yellow solu-
tion, that was stirred for a further 14 h under ambient conditions.
Degassed water (1.8 mL, 100.00 mmol) was subsequently added
dropwise with a syringe causing the formation of a clear yellow
solution and a white precipitate. The mixture was filtered through
Celite and the volatiles were removed to afford a pale-yellow liquid
that was used without further purification. Yield: 11.75 g (70%).
δ ϭ 3.37 (m, 4 H, Cy), 1.91Ϫ1.24 (m, 44 H, Cy), 0.33 (s, 36 H,
SiMe3) ppm.
An analogous reaction procedure using iPrNϭCϭNiPr afforded a
colourless crystalline material that analysed for 3.
[Zn{Me2NC(NiPr)2}Me]2 (5): A solution of 1 (1.00 g, 5.84 mmol)
in toluene (30 mL) was added at room temperature to a solution
of dimethylzinc (2.9 mL of a 2  solution in toluene, 5.80 mmol)
that had been further diluted by 50 mL of toluene. Evolution of a
gas was observed and the resultant clear yellow solution was stirred
under ambient conditions for 12 h. Removal of the volatiles af-
forded a pale-yellow solid that could be used without further purifi-
cation. Analytically pure samples were obtained by crystallisation
from hexane at Ϫ30 °C. Yield 0.98 g (67%). C10H23N3Zn (501.37):
calcd. C 47.91, H 9.25, N 16.76; found C 47.73, H 9.37, N 16.86.
3
1H NMR (CDCl3, 298 K): δ ϭ 3.37 (sept, JH,H ϭ 6.4, 1 H,
3
CHMe2), 3.25 (sept, JH,H ϭ 6.3, 1 H, CHMe2), 2.67 (s, 6 H,
3
3
NMe2), 1.03 (d, JH,H ϭ 6.3, 6 H, CHMe2), 1.02 (d, JH,H ϭ 6.3,
6 H, CHMe2) ppm; NH signal not observed. 13C NMR (CDCl3,
298 K): δ ϭ 155.8 (CN3), 47.3 (CHMe2), 45.9 (CHMe2), 39.0
(NMe2), 25.1 (CHMe2), 23.6 (CHMe2) ppm.
3
1H NMR (C6D6, 298 K): δ ϭ 3.41 (sept, JH,H ϭ 6.1, 2 H,
CHMe2), 2.47 (s, 6 H, NMe2), 1.12 (br. d, 12 H, CHMe2), Ϫ0.12
(s, 3 H, ZnMe) ppm. 13C NMR (C6D6, 298 K): δ ϭ 166.5 (CN3),
47.2 (CHMe2), 40.0 (NMe2), 25.7 (br., CHMe2), Ϫ11.2 (ZnMe)
(Me3Si)2NC(NiPr)(NHiPr) (2): Compound 2 was synthesised using
a similar procedure to that outlined for the synthesis of 1, using
lithium bis(trimethylsilyl)amide (2.50 g, 14.94 mmol), diisopro-
pylcarbodiimide (1.89 g, 14.94 mmol) and water (0.27 g,
14.94 mmol). The compound was isolated as a colourless liquid
ppm. MS (EIϩ): m/z
ϭ
404 [{Me2NC(NiPr)2}2Zn]ϩ, 360
[{Me2NC(NiPr)2}2Zn Ϫ NMe2]ϩ, 234 [{Me2NC(NiPr)2}Zn]ϩ.
[Zn(OAr)Me·{Me2NC(NiPr)(NHiPr)}] (6): A solution of 2,6-di-
tert-butylphenol (0.41 g, 2.00 mmol) in toluene (20 mL) was added
1
and was used without further purification. Yield: 3.21 g (75%). H
3
NMR (CDCl3, 298 K): δ ϭ 3.79 (br. sept, JH,H ϭ 6.0, 1 H,
dropwise at room temperature to
a solution of 5 (0.50 g,
3
CHMe2), 3.64 (br. sept, JH,H ϭ 5.6, 1 H, CHMe2), 1.08 (br. d,
2.00 mmol) in toluene (30 mL). The resultant clear, pale-yellow
solution was stirred at room temperature for 12 h, after which time
the volatile component was removed in vacuo to afford a pale-
yellow solid. Crystallisation from hexane afforded 6 as colourless
crystals. Despite repeated crystallisations, a persistent, unidentified
impurity was observed during the preparation of 6. As such, accu-
rate analysis was not obtained and the NMR spectroscopic data
are reported for the major component, which corresponds to 6.
Yield: 0.43 g (46%) calculated for 6. C24H45N3OZn (457.00): calcd.
3JH,H ϭ 5.9, 6 H, CHMe2), 1.04 (br. d, 3JH,H ϭ 5.8, 6 H, CHMe2),
1.15 (s, 18 H, SiMe3) ppm; NH signal not observed. 13C NMR
(CDCl3, 298 K): δ ϭ 147.5 (CN3), 47.3 (CHMe2), 42.0 (CHMe2),
24.9 (CHMe2), 22.7 (CHMe2), 1.6 (SiMe3) ppm.
[Zn{(Me3Si)2NC(NiPr)2}2] (3). Method 1: A solution of dimeth-
ylzinc (0.9 mL of a 2  solution in toluene, 1.80 mmol) was further
diluted with an additional 20 mL of toluene and cooled to Ϫ78 °C.
A solution of 2 (0.50 g, 1.74 mmol) in toluene (40 mL) was added
dropwise with a cannula and the resultant colourless solution was
allowed to warm to room temperature and stirred for 14 h. The
mixture was filtered and cooled to Ϫ30 °C to afford colourless
crystals of [Zn{(Me3Si)2NC(NiPr)2}2]. Yield: 0.45 g (48%).
C26H64N6Si4Zn (638.56): calcd. C 48.91, H 10.10, N 13.16; found
C 48.99, H 9.99, N 13.07. 1H NMR (C6D6, 298 K): δ ϭ 3.70 (sept,
1
C 63.07, H 9.92, N 9.19; found C 56.74, H 9.39, N 6.73. H NMR
3
(C6D6, 298 K): δ ϭ 7.46 (d, JH,H ϭ 7.8, 2 H, m-C6H3), 6.92 (t,
3
3JH,H ϭ 7.7, 1 H, p-C6H3), 3.29 (sept, JH,H ϭ 6.5, 1 H, CHMe2),
3.08 (br. sept, 1 H, CHMe2), 2.19 (br. s, 6 H, NMe2), 1.72 (s, 18
3
3
H, CMe3), 1.17 (br. d, JH,H ϭ 6.4, CHMe2), 0.99 (br. d, JH,H
ϭ
6.0, CHMe2), Ϫ0.33 (s, 3 H, ZnMe) ppm. 13C NMR (C6D6,
298 K): δ ϭ 165.7 (CN3), 139.0 (C), 128.3 (C), 125.3 (CH), 115.9
(CH), 50.1 (br., CHMe2), 45.6 (CHMe2), 39.8 (NMe2), 35.5
(CMe3), 32.4 (CMe3), 24.9 (CHMe2), 22.7 (CHMe2), Ϫ8.8 (ZnMe)
ppm. IR (nujol mull, KBr): ν˜ ϭ 3234 (m), 3140 (m), 1596 (s), 1518
(s), 1404 (s), 1365 (s), 1349 (m), 1318 (m), 1254 (s), 1243 (s), 1198
(w), 1183 (w), 1171 (w), 1136 (w), 1122 (m), 1102 (m), 1064 (w),
1047 (m), 975 (w), 938 (w), 881 (w), 853 (m), 814 (m), 752 (s), 686
3
3JH,H ϭ 6.1, 4 H, CHMe2), 1.17 (d, JH,H ϭ 6.2, 24 H, CHMe2),
0.30 (2, 36 H, SiMe3) ppm. 13C NMR (C6D6, 298 K): δ ϭ 165.2
(CN3), 44.7 (CHMe2), 27.0 (CHMe2), 2.3 (SiMe3) ppm. MS (EIϩ):
m/z ϭ 636 [M]ϩ, 621 [M Ϫ Me]ϩ. Method 2: A solution of 2
(0.37 g, 1.29 mmol) in toluene (20 mL) was added dropwise at room
temperature to a toluene (25 mL) solution of [Zn{N(SiMe3)2}2]
(0.50 g, 1.29 mmol) with a cannula. No visible change was ob-
served. The reaction mixture was stirred at room temperature for
18 h, after which time the volatiles were removed to afford a sticky
(m), 634 (w) cmϪ1
.
[{Zn(OAr)}2(µ-{Me2NC(NiPr)2})(µ-NMe2)] (7): 2,6-Di-tert-bu-
tylphenol (0.41 g, 2.00 mmol) in toluene (20 mL) was added drop-
wise at room temperature to a solution of 5 (0.50 g, 2.00 mmol) in
toluene (30 mL). The resultant clear, pale-yellow solution was
heated at 75 °C for 3 d, after which time a clear colourless solution
had formed. The volatiles were removed to afford a waxy solid
that was redissolved in hexane. Cooling of this solution to Ϫ30 °C
afforded a small number of colourless crystals suitable for an X-
1
white solid. H NMR spectra indicated a 1:1 mixture of 3 (ident-
ified from the chemical shift values in comparison with an auth-
entic sample) and unchanged [Zn{N(SiMe3)2}2]. No further purifi-
cation was attempted.
[Zn{(Me3Si)2NC(NCy)2}2] (4):
A solution of CyNϭCϭNCy
(0.27 g, 1.29 mmol) in toluene (25 mL) was added dropwise to a
stirred solution of [Zn{N(SiMe3)2}2] (0.50 g, 1.29 mmol) in toluene
(25 mL) at room temperature. No visible change was observed, and
the resultant colourless solution was stirred at ambient conditions
for 16 h. Removal of the volatiles in vacuo afforded crude 2 as a
white solid that was crystallised from toluene at Ϫ30 °C as analyti-
cally pure colourless crystals. Yield: 0.32 g (62%) based on CyNϭ
1
3
ray analysis. H NMR (C6D6, 298 K): δ ϭ 7.36 (d, JH,H ϭ 7.7, 4
H, m-C6H3), 6.86 (t, 3JH,H ϭ 7.7, 2 H, p-C6H3), 2.98 (sept, 3JH,H ϭ
6.4, 2 H, CHMe2), 2.73 (s, 6 H, NMe2), 2.16 (s, 6 H, NMe2), 1.63
(s, 36 H, CMe3), 0.87 (d, JH,H ϭ 6.4, 12 H, CHMe2) ppm. 13C
3
NMR (C6D6, 298 K): δ ϭ 169.8 (CN3), 165.6 (C), 138.2 (C), 125.1
(CH), 115.9 (CH), 48.6 (CHMe2), 44.2 (NMe2), 40.2 (NMe2), 35.6
CϭNCy. C38H80N6Si4Zn (798.81): calcd. C 57.14, H 10.09, N
1
10.52; found C 57.05, H 10.09, N 10.56. H NMR (C6D6, 298 K): (CMe3), 31.8 (CMe3), 26.1 (CHMe2) ppm.
2670
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Zinc Guanidinate Complexes
FULL PAPER
Table 4. Crystal structure and refinement data for 3Ϫ8
3
4
5
6
7
8
Empirical formula
Formula mass
C26H64N6Si4Zn
638.56
C38H80N6Si4Zn
798.81
C20H46N6Zn2
501.37
C24H45N3OZn
457.00
C39H68N4O2Zn2
755.71
C26H60N8Si4Zn2
727.92
Temperature [K]
Wavelength [A]
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
173(2)
0.71073
˚
Crystal size [mm]
Crystal system
Space group
0.40 ϫ 0.35 ϫ 0.20 0.20 ϫ 0.15 ϫ 0.10 0.30 ϫ 0.10 ϫ 0.05 0.30 ϫ 0.30 ϫ 0.25 0.10 ϫ 0.10 ϫ 0.02 0.40 ϫ 0.30 ϫ 0.05
monoclinic
C2/c (No. 15)
17.3585(4)
8.7203(2)
26.1372(6)
90
94.137(1)
90
3946.11(16)
4
orthorhombic
Fddd (No. 70)
9.8360(3)
30.3263(9)
32.0293(9)
90
90
90
9554.0(5)
8
orthorhombic
Fdd2 (No. 43)
21.5372(11)
27.5586(15)
8.8220(4)
90
90
90
5236.2(5)
8
triclinic
P1 (No. 2)
monoclinic
P21/c (No. 14)
9.7245(2)
27.4359(6)
16.0939(2)
90
104.185(1)
90
4162.94(14)
4
monoclinic
P21/c (No. 14)
10.6494(2)
11.4158(4)
15.7858(4)
90
100.008(1)
90
1889.90(7)
2
¯
˚
a [A]
10.5249(2)
10.5294(2)
13.2813(3)
78.008(1)
73.935(1)
66.487(1)
1288.99(5)
2
˚
b [A]
˚
c [A]
α [°]
β [°]
γ [°]
3
˚
V [A ]
Z
Dc [Mg·mϪ3
]
1.08
1.11
1.27
1.18
1.21
1.28
Absorption coefficient [mmϪ1
]
0.77
0.65
1.85
0.97
1.19
1.42
θ range for data collection [°] 3.78 to 25.60
3.85 to 25.00
10132
4.28 to 25.02
14344
3.79 to 24.72
16240
3.72 to 25.02
61410
3.80 to 24.73
11043
Reflection collected
Independent reflections
Reflection with I Ͼ 2σ(I)
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices
11770
3509 (Rint ϭ 0.060) 2079 (Rint ϭ 0.0481) 2233 (Rint ϭ 0.065) 4347 (Rint ϭ 0.050) 7332 (Rint ϭ 0.082) 3205 (Rint ϭ 0.048)
2836
3509/0/168
1.017
R1 ϭ 0.043,
wR2 ϭ 0.102
R1 ϭ 0.059,
wR2 ϭ 0.110
0.28/Ϫ0.30
1714
2079/0/113
1.060
R1 ϭ 0.044,
wR2 ϭ 0.099
R1 ϭ 0.058,
wR2 ϭ 0.107
0.58/Ϫ0.47
2077
2233/1/128
1.050
R1 ϭ 0.038,
wR2 ϭ 0.087
R1 ϭ 0.044,
wR2 ϭ 0.090
0.37/Ϫ0.38
3700
4347/0/269
1.018
R1 ϭ 0.035,
wR2 ϭ 0.075
R1 ϭ 0.046,
wR2 ϭ 0.079
0.27/Ϫ0.36
5443
7332/0/428
1.060
R1 ϭ 0.051,
wR2 ϭ 0.105
R1 ϭ 0.079,
wR2 ϭ 0.117
0.69/Ϫ0.42
2780
3205/0/181
1.044
R1 ϭ 0.030,
wR2 ϭ 0.069
R1 ϭ 0.039,
wR2 ϭ 0.072
0.34/Ϫ0.42
[I Ͼ 2σ(I)]
R indices (all data)
Ϫ3
˚
Largest diff. peak/hole [e·A
]
[Zn(hpp){N(SiMe3)2}]2 (8): A solution of hppH (0.18 g, 1.29 mmol) www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
in toluene (20 mL) was added dropwise to a toluene solution
(25 mL) of [Zn{N(SiMe3)2}2] (0.50 g, 1.29 mmol) at room tempera-
ture. No visible change was observed. The solution was stirred at
room temperature for 18 h, after which time the volatile compo-
nents were removed in vacuo to afford [Zn(hpp)N(SiMe3)2]2 as a
white powder. Yield: 0.43 g (90%). Analytically pure samples for
applications in polymerisation experiments were obtained by
recrystallisation of a sample of 7 from hexane at Ϫ30 °C, affording
colourless parallelepipedal crystals. C26H60N8Si4Zn2 (727.92):
calcd. C 42.90, H 8.31, N 15.39; found C 43.05, H 8.20, N 15.27.
1H NMR (C6D6, 298 K): δ ϭ 3.22 (m, 8 H, hpp-CH2), 2.52 (t,
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; Fax: ϩ 44 -1223-336033; E-mail: deposit@ccdc.cam.ac.uk].
Acknowledgments
We thank the University of Sussex for additional financial support.
We also wish to acknowledge the use of the EPSRC’s Chemical
Database Service at Daresbury.
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Early View Article
Published Online May 5, 2004
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2672
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 2662Ϫ2672